No, it’s not official. The demo team simply chose VContainer to plug the shortcomings of the default engine architecture. However, it is a nod towards the solution as being a viable one.
No. Unity most likely picked it because it’s the only actively developed free open source dependency injector. Like you noticed Zenject (and the fork of it Extenject) have been abandoned. There is the paid asset Init(args) but they wouldn’t be able to freely distribute that with a demo project.